The purpose of this study is to compare the concentration of \[6R\] 5,10-methylene-THF, 5-formyl-THF, 5-methyl-THF and THF in the tumor, in adjacent mucosa and in plasma in patients with colon cancer receiving two different does of Modufolin® and Levoleucovorin (Isovorin®), respectively (60 and 200 mg/m2).
